1. Field of the Invention
This invention relates to an apparatus and method for measuring sliding friction between two materials. More specifically the invention relates to an apparatus in which a sphere of one material having a bore through one diameter is held within a cavity in a cylinder having a bore; caused to spin within that cavity and the time measured for the spherical bore to align with the cylindrical bore. The time measured is inversely proportional to the coefficient of sliding friction between the material of the sphere and the material of the cavity.
2. Description of Prior Art.
When two bodies in contact are moved relative to one another, the coefficient of sliding friction is given by the ratio of the force required to sustain the motion divided by the normal component of the force holding the two bodies in contact. Measurement of the coefficient of sliding friction is typically accomplished in one of three ways within prior art. For high loads and low sliding speeds, the method shown schematically in FIG. 1 is used. Friction is measured between a flat sliding lower surface and a stationary upper surface called a rider. Movement of the lower surface sets up a frictional force between the two surfaces and the amount of the deflection of the rider is proportional to that force. For high speeds and lighter loads, the rider is pressed against the rim of a revolving disk instead of a flat plate. These devices require careful control of the motion between the rider and the moving surface as well as careful control and measurement of the two pertinent forces. These devices are characteristically complex and subject to inaccuracies due to difficulty in obtaining precise motion control and force or torque measurements while the apparatus is in motion. Other complicating limitations are the existence of static friction, mechanical vibrations and severe frictional heating.
A third method within prior art, shown in FIG. 2, involves suspending a steel ball in the magnetic field of a solenoid with vertical stability maintained by means of a photoelectric feedback system. The freely-suspended ball is then accelerated by a rotating magnetic field of constant frequency in a low-pressure atmosphere between three flat friction pads. Bringing the pads into contact with the steel ball causes rotational deceleration according to the relation: ##EQU1## where .mu..sub.s =coefficient of sliding friction
F=friction force PA1 N=Normal force PA1 I=moment of inertia of the ball PA1 .omega.=Angular deceleration of the ball PA1 R=radius of the ball
However this technique, like the others involves complex machinery and difficult, precise motion control and force or torque measurments while in operation. A further limitation is that at least one of the materials for which the measurement is made must be magnetic.
The present invention is of simple construction, requires no complex control of motion, force or torque measurements and requires only the measurement of angular distance traversed and the time elapsed for this increment of motion. Further, the present invention has no problems of mechanical vibration and permits a very great selection of materials, speed of relative motion, surface finishes and operating conditions such as temperature and lubrication of the surfaces.